1. Field of the Invention
The present invention relates to a capacitive touch screen, and more particularly, to a capacitive touch screen that reduces the effect of negative touches.
2. Description of the Prior Art
Referring to FIG. 1A, when a driving signal D is passing through a driven conductive strip, a signal I may flow from a first finger A to another finger B of the same hand, such that during scanning of sensing information SI, signal variations in mutual-capacitive coupling will be sensed on conductive strips corresponding to fingers A and B, such as touch-related sensing information SA and SB as shown, respectively. It can be seen from FIG. 1A that the directions in which the touch-related sensing information SA and SB vary are opposite to each other, that is, the signals are opposite to each other.
The touch-related sensing information SA represents the variations in capacitive coupling at an intersecting region between a sensed conductive strip corresponding to the location of the first finger A and a driven conductive strip. In this case, a real touch exists. Similarly, the touch-related sensing information SB represents the variations in capacitive coupling at an intersecting region between a sensed conductive strip corresponding to the location of the second finger B and a driven conductive strip. However, the intersecting region represented by the touch-related sensing information SB is actually not touched, thus resulting in a so-called “negative (unreal) touch”, i.e. a “phantom touch”. In the following descriptions, a signal flowing out of a conductive strip due to the capacitive coupling of the first finger A is called a real-touch signal, while a signal flowing out of a conductive strip due to the capacitive coupling of the second finger B is called a negative-touch signal. Thus, the variations in capacitive couplings corresponding to the real-touch and negative-touch signals detected on the conductive strips are real touch-related sensing information and negative touch-related sensing information, respectively.
Referring to FIG. 1B, when the first finger A and the second finger B are on the same or nearby sensed conductive strip(s), the corresponding touch-related sensing information SA and SB will cancel each other as the signals are opposite to each other, thus reducing the signal. When the magnitudes of the touch-related sensing information SA and SB are close to each other, the resulting signal may be too small to be determined as a real touch. In the following descriptions, the situation in which the variations in the detected capacitive coupling of the real touch are distorted due to the fact that the negative and real touches are in proximity to each other is called a “negative-touch effect”.
In the above example, the first finger A and the second finger B are capacitively coupled to the conductive strips via an insulating surface layer. The thinner the insulating surface layer, the greater the negative-touch effect. In other words, the greater the distortion of the variation in the detected capacitive coupling of the real touch. Furthermore, the more negative touches caused by the second finger B, the larger the total number of negative-touch signals, and the greater the distortion of the variation in the detected capacitive coupling of the real touch, even to the extent that an original real touch-related sensing information is regarded as a negative touch-related sensing information. In other words, in the worst-case scenario in which all the signals from the second finger B and the signal from the first finger A are on the same detected conductive strip, the negative-touch effect is greatest at this time. Needless to say, in mutual-capacitive coupling, tolerance to the negative-touch effect determines if the location of a real touch can be correctly detected and the number of locations of real touches that can be detected at the same time.
The problem of negative-touch effect is more severe in portable devices. This is because the ground that is in contact with a portable device is different from the ground that is in contact with a human body. In order to meet market demands, thinner portable devices are desired, and as a result of this, the capacitive touch screen is also made thinner. The capacitive touch screens are usually arranged above the display, so noise coming from the display constantly interferes with the capacitive touch screen. In order to reduce interference, the most direct way is to add a rear shielding layer to the back of the capacitive touch screen (the portion nearer to the display), which connects to a ground potential to eliminate noise coming from the display. However, the addition of the rear shielding layer inevitably increases the thickness of the capacitive touch screen, and which does not meet the requirements of the markets.
Another approach that does not require the addition of a rear shielding layer while reducing the interference of the noise from the display is to arrange conductive strips that will be provided with the driving signal (driven conductive strips) on a lower layer, and sensed conductive strips on an upper layer in a double ITO (DITO) structure, wherein the driven conductive strips cover most of the display. Except for the conductive strip that is being provided with the driving signal, all the other strips are coupled to ground, thus creating an effect similar to a rear shielding layer. Since the sensed conductive strips are on the upper layer, in order to reduce the negative-touch effect, the thickness of the insulating surface layer thus cannot be effectively made thinner. When the insulating surface layer is made of a glass material, the distance between a sensed conductive strip and a finger tip needs to be kept at about 1.1 mm or more. Even if a plastic material is adhered to support the glass, the distance between a sensed conductive strip and a finger tip needs to be kept at about 0.7 mm or more. With such strict restrictions to the thickness of the insulating surface layer, the remaining solution is to reduce the thickness of an insulating intermediate layer between the driven conductive strips and the sensed conductive strips.
Compared to a DITO structure, the thickness of the insulating surface layer in a single ITO (SITO) structure also faces the same limitation. However, there is no insulating intermediate layer, so the overall thickness is much smaller than the DITO structure, but the rear shielding effect similar to the one discussed above is also lost. If noise interference cannot be effectively eliminated, then it is better to arrange the SITO structure inside the display (in cell). If it is arranged above the display, then the provision of a rear shielding layer may become a necessity.
Noise interference arising from the display hinders the ability to correctly determine the location of a real touch, while the negative-touch effect affects the ability to correctly determine the locations of multiple real touches. Obviously, in order to reduce the thickness of the capacitive touch screens, one needs to consider the distance between the sensed conductive strips and the finger tip, and moreover, how to eliminate the noise interference coming from the display.
From the above it is clear that prior art still has shortcomings. In order to solve these problems, efforts have long been made in vain, while ordinary products and methods offering no appropriate structures and methods. Thus, there is a need in the industry for a novel technique that solves these problems.